The invention relates to inverters in general and more particularly to an inverter for induction heating. Such an inverter supplies high frequency alternating current to the induction heating coil which forms a tank circuit that has varying electrical characteristics according to the material and size of the workpiece and the temperature of the workpiece being heated. The load in the induction heating installation has an inductive component, a capacitive component and a resistive component. Consequently, it is susceptible to frequency changes and presents a load that varies drastically. In recent years, there has been a substantial amount of work devoted to the development of a high power solid-state power supply for driving an induction heating load. Such devices generally convert D.C. current to an alternating current which flows through the load. One of the most common of these devices is a solid-state inverter having a constant current supplied from a D.C. source, which current is alternately switched through the load in different directions by two distinct sets of switching devices, generally SCR's. This type of solid-state device has been used in tandem with a power rectifier which converts available three phase alternating current into D.C. current. This D.C. current is then directed to the inverter which changes the direct current into a single phase alternating current of a controllable high frequency. Frequency of the inverter is controlled by the rate at which gating signals are provided to the SCR's. One form of such inverters is a parallel-compensated inverter which is attached to a constant current source and is well known in the induction heating art.
In order for the SCR's to be commutated to the OFF condition, it is necessary to apply a reverse voltage across the individual SCR's for a time which is referred to as the turn-off time of the SCR's or other switching devices. This switching time varies according to the particular type of switching device used. As the frequency increases, there is less time available for commutating the individual switching devices or SCR's. Consequently, with high frequency inverters of 1.0 KHertz and higher, relatively precise switching devices are required. Such devices are expensive. Indeed, commercial SCRs can not exceed a preselected high frequency when a large margin of safety is provided to assure turn-off time. Thus, as inverters of the type described above are used for high frequency heating, generally required in induction heating, the SCR's become expensive and there is a relatively low maximum frequency to be obtained without modifying the well known inverter circuits. For this reason, controlling the gating pulses or gating signals for the SCR's has been the subject of substantial developmental work. Gating is generally controlled by monitoring the phase relationship of the voltage and current through the load and then adjusting this relationship so that sufficient turn-off time is assured. This concept limits the versatility of the inverter, necessitates expensive SCR's, requires an expensive choke to assure continuous current flow and generally complicates the inverter circuit itself.
Another disadvantage of power inverters, which is especially serious when used for widely varying loads, is that such inverter for supplying an induction load is difficult to start. It is usually impossible to start the inverter by merely providing gating pulses to the SCR's in the inverter in a manner similar to the steady-state condition. At start up, there is no energy in the tuned load for commutating the thyristors or SCR's. This starting problem is further compounded when the load is spaced from the inverter so that substantial inductance is created by connecting leads between the inverter and the load. Because of these difficulties, a substantial amount of work has been devoted to providing an arrangement for starting a power inverter used in the induction heating art. Circuits have been suggested for causing initial current oscillation through the load during the starting cycle. Some circuits involve switching of special capacitors across the load. Circuits have been provided to initially charge the tuned load to build up oscillations in the load before the gating of the inverter is started. Since this starting concept requires matching of an auxiliary circuit with the characteristics of the load, such concept could not be used for a wide range of load conditions. The most widely adopted concept is the provision of a smoothing inductor or reactor between the input D.C. supply and the thyristors or SCR's of the inverter. A precharging current is then directed through the inverter into the tuned load to shock the load into oscillations when the SCR's are gated. This technique required a relatively large reactor between the rectifier and inverter which is expensive and substantially adds to the cost of the device. In addition, the characteristics of the load negated this concept as an effective means for allowing start-up of a power inverter of the type used in induction heating.